Typo: fix a typo in layer.md
Typo: fix a typo in layer.md

Signed-off-by: Lei Jitang <9ac444d2b5df3db1f31aa1c6462ac8e9e2bde241@huawei.com>

mattspencer-arm/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,opencontainers/image-spec,xiekeyang/image-tools,jonboulle/image-spec,xiekeyang/image-tools,vbatts/oci-image-spec,opencontainers/image-spec,opencontainers/image-tools,jonboulle/image-spec,opencontainers/image-tools